1. Field of the Invention
The present invention generally relates to an exhaust gas re-circulating (EGR) apparatus and, more particularly, to an exhaust gas re-circulating apparatus for returning a portion of exhaust gas in a multi-cylinder engine to a respective cylinder so as to reduce the volume of NOX in the exhaust gas.
2. Description of the Related Art
FIG. 3 is a schematic diagram showing the construction of a prior-art exhaust gas re-circulating apparatus disclosed in the Japanese Laid-Open Utility Model Application No. 4-44462. Referring to FIG. 3, the apparatus comprises an engine 1 provided with a plurality of cylinders, an intake manifold 1A for the engine 1, an exhaust manifold 1B for the engine 1, an intake pipe 2 for the intake manifold 1A, intake branch pipes 3, 4, 5 and 6 branching off of the intake pipe 2 and connected to respective intake ports of the cylinders of the engine 1, exhaust branch pipes 7, 8, 9 and 10 of the exhaust manifold 1A. The exhaust branch pipes 7, 8, 9 and 10 are connected to respective exhaust ports of the cylinders of the engine 1. An exhaust pipe 11 of the exhaust manifold 1B collects the exhaust gas from the exhaust branch pipes and guide the same to the outside.
The apparatus further comprises an exhaust gas re-circulating passage 12 branching off of the exhaust pipe 11 and returning a portion of the exhaust gas to a combustion chamber of the engine 1, branch pipes 13, 14, 15 and 16 branching off of the exhaust gas re-circulating passage 12 and connected to the intake branch pipes 3, 4, 5 and 6 of the intake manifold 1A so as to correspond to respective cylinders of the engine 1, and exhaust gas re-circulating valves (EGR valves) 17 provided at respective joints between the branch pipes 13, 14, 15 and 16 and the exhaust gas re-circulating passage 12. The exhaust gas re-circulating valves 17 of the branch pipes 13, 14, 15 and 16 are removed from respective joints with the intake branch pipes 3, 4, 5 and 6 by different distances.
An O2 sensor 18 is provided in each of the intake branch pipes 3, 4, 5 and 6 so as to measure the volume of intake air taken in by the respective cylinder of the engine 1. A temperature sensor 19 is provided in each of the intake branch pipes 3, 4, 5 and 6 so as to measure the temperature of the intake air. An ECU (controller) 20 receives a signal from the O2 sensor 18 and the temperature sensor 19 so as to control the opening and closure of each of the exhaust gas re-circulating valves 17. In addition to the signal from the O2 sensor 18 and the temperature sensor 19, the ECU 20 is supplied with data including clearance of a throttle lever from a base position, the revolution of the engine and the temperature of the engine cooling water.
A description will now be given of the operation of the prior-art gas re-circulating apparatus.
When the engine 1 is being operated, the control signal from the ECU 20 causes the exhaust gas re-circulating valve 17 to open so that a portion of the exhaust gas flowing in the exhaust pipe 11 of the exhaust manifold 1B is mixed with the intake air in the branch pipes 3, 4, 5 and 6 of the intake manifold 1A via the branch pipes 13, 14, 15 and 16 of the exhaust gas re-circulating passage 12, and returned to the respective cylinder of the engine 1. If the volume of re-circulated air differs from cylinder to cylinder, combustion occurs in the cylinders in different manners. The exhaust gas re-circulating valve 17 is controlled to open slightly or extensively in accordance with the signal from the ECU 20 produced on the basis of the signals from the O2 sensor 18 and the temperature sensor 19, so as to correct the differences in the manner of combustion. Thus, generation of NOX is restrained by controlling the volume of exhaust gas re-circulated to the cylinders of the engine 1.
With the above-described construction of the prior-art exhaust gas re-circulating apparatus, generation of NOX in the exhaust gas is controlled to a certain extent. However, the length of the branch pipes 13, 14, 15 and 16 of the exhaust gas re-circulating pipe 12, that is, the length of the re-circulation path from the exhaust gas re-circulating valve 17 to the respective cylinder of the engine 1 differs from cylinder to cylinder. This causes the volume of exhaust gas re-circulated to the respective cylinder to vary from cylinder to cylinder when the exhaust gas re-circulating valves 17 of the cylinders are similarly opened, thereby producing a variation in combustion performance from cylinder to cylinder. Such a variation is unfavorable in terms of the EGR effect provided as a result. The exhaust gas re-circulating valve 17 is provided at a respective branch of the branch pipes 13, 14, 15 and 16 of the exhaust gas re-circulating passage 12 and is removed from the intake manifold 1A. Thus, the EGR response is relatively poor. In the case of a diesel engine, black smoke may be produced.